1. Technical Field
The present disclosure relates generally to impedance altering circuits, and more particularly relates to circuits and methods for increasing the impedance of a circuit by balancing the current flow into and out of the circuit.
2. Description of the Related Art
Circuits which provide for a precise increase in a given impedance are well known in the art.
One effect known to alter the equivalent impedance at a subject node is frequently described in connection with the capacitive grid-cathode impedances in an electron tube as related to the plate-cathode capacitive impedance. This is known as the Miller Effect. In such circuits the equivalent input impedance is a function of an impedance coupling the subject node to another node whose voltage potential is proportional to a voltage potential present at the subject node. The effective impedance at the subject node is an accurately related to this proportion.
Referring to FIG. 1, a reference impedance element 10 having a value Z.sub.A, is subject to an applied voltage from voltage source 12 at node 14. A voltage controlled voltage source circuit (VCVS) circuit 16 is included and has an input terminal coupled to node 14 and an output terminal coupled to impedance element 10. The VCVS 16 has a voltage gain G describing the voltage produced at node 18 by the VCVS with respect to a voltage V.sub.X at node 14. Accordingly, the effective impedance at node 14, Z.sub.effective, is the ratio of the voltage V.sub.X divided by the current flowing through voltage source 12, I.sub.VX. This proportion is related to the gain G as follows: EQU Z.sub.effective =V.sub.X /I.sub.VX =Z.sub.A /(1-G) (1)
The principal drawback of such a circuit is that the impedance upon which the effective impedance is based must be connected to one end of the VCVS 16. For this reason, circuits which increase the impedance between two given nodes in a circuit cannot be implemented without a substantially more costly circuit. Such circuits, therefore, are practically limited to adjusting the effective impedance at only a single circuit node, with the other node subject to the short circuit impedance of the VCVS circuit 16. Hence, such circuits are typically not symmetrical with respect to the two nodes 14, 18 of impedance element 10.
Circuits as illustrated in FIG. 1, are also identified as bootstrap circuits, as exemplified by U.S. Pat. No. 5,568,561 to Whitlock. The Whitlock patent describes a circuit which utilizes the principle of the Miller Effect described above, where the factor G is determined to be very nearly one by virtue of a unity gain buffer circuit.
Accordingly, it is an object to alter the effective impedance between two nodes in a circuit between which a given fixed impedance element exists without interjecting any additional circuits in series with the fixed impedance.
It is another object to provide enhanced control of a circuit impedance using a potentiometer in a manner that is more stable over temperature than a conventional variable resistor, and which is capable of producing a controllable impedance with an accuracy and temperature stability on the order of the fixed components that constitute the circuit.
It is a further object to modify an impedance of a subject circuit impedance with a circuit that cancels the effect of the subject circuit impedance on external connected circuitry.
It is yet another object of the present invention to implement precision voltage dividers with smaller resistance values in such a way that precision voltage division can be performed without loading the source voltage potential that is so divided.
It is still another object to enhance the implementation of differential amplifier circuits whose circuits are isolated with respect to other related circuits in a way that enhances not only the input impedance characteristics but also the output impedance characteristics in such cases.
It is yet another object to generally increase the input impedance of a given circuit without altering the function of any existing impedances in or related to the given circuits input impedance.
It is yet a further object to provide enhanced levels of isolation between an input and an output of a signal conditioning circuit without compromising the performance of such circuits in any way.
It is yet another objective to increase the dynamic isolation of a practical current source.
It is yet a further object to perform accurate impedance enhancement over a range of frequencies.